The Great Lakes Plain meets the Appalachian Plateau in the extreme northwestern part of the state. The Appalachian Plateau stretches across the western and northern sections of Pennsylvania and covers more than half the area of the state. The Allegheny Mts. line the eastern edge of the plateau and run southwest to northeast, overlooking the Great Appalachian Valley. The Jacks, Tuscarora, and Blue Mts. comprise a ridge and valley section bordered by the Great Appalachian Valley to the southeast and east. The Piedmont Plateau gives way to the Atlantic Coastal Plain in the extreme southeastern portion of the state.
In the east Pennsylvania is drained by the Delaware and the Susquehanna river systems; in the west by the Allegheny and the Monongahela rivers, which join at Pittsburgh to form the Ohio River; and in the central part by the West Branch of the Susquehanna, which crosses the state and empties into Chesapeake Bay. These turbulent streams and rivers have cut beautiful water gaps, natural passageways for roads and rail lines.
The great forests and lush vegetation that once covered the entire state were transformed during the Carboniferous period into deposits of anthracite coal in the northeast and extensive bituminous beds in the west. Large areas of woodland remain and, in some isolated sections, have retained an almost primitive wildness. Of the many historic sites and parks that have been preserved, those under federal ownership include Fort Necessity National Battlefield, Gettysburg National Military Park, and Independence and Valley Forge national historical parks (see National Parks and Monuments, table). Harrisburg, the state capital, is located between the metropolitan areas of Philadelphia, the largest city, and Pittsburgh.
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